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E. coli bacterium, artwork. The Escherichia coli bacterium is a Gram-negative bacillus (rod-shaped bacterium). It commonly has a single long flagellum (thin thread-like structure) that is used for movement. However, this strain has numerous flagella, giving it greater mobility. E. coli is a normal inhabitant of the human intestine however, under certain conditions, its numbers may increase, causing infection.